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What's an ADU?

6a00d8341c67ce53ef01310f51699b970c-800wi Sometimes referred to as a mother-in-law apartment or a granny flat, an accessory dwelling unit (ADU) is a secondary living space on a property that would normally accommodate only one dwelling unit. This could be above a detached garage, in a basement, in a separate structure or one of many other arrangements.

For a long time ADUs were actually illegal to build in many communities and in some places they still are in the name of "protecting" single-family neighborhoods.

There are a lot of benefits to ADUs

  • Increase a community's housing supply.
  • Offer affordable housing options for low to medium income residents, young people fresh out of school, or older people wanting to scale down.
  • Offer temporary residences for visitors.
  • Being able to rent the ADU offers homeowners another source of income.
  • Enhances neighborhood business economics by bringing more people into the neighborhood and more disposable income to homeowners.
  • People who come on hard times due to unforseen circumstances are able to move out of the main residence and in to the ADU. This allows them to rent the main residence and increase their income.
  • Give an option for elderly homeowners who are no longer able to manage the main home to stay on the property and in their community.
  • Inexpensive way to increase density in neighborhoods without the need for additional infrastructure.
  • Effectively improves the affordability of housing for both homeowners and renters at all stages in their lives
  • Maximize use of existing public infrastructure and services and reduce the pressure on open space and farmlands from sprawling development.
  • And many more...

What are the laws concerning ADUs in Mountlake Terrace?

Here are the requirements concerning ADUs in Mountlake Terrace:

  • Owner occupied: An owner of the property, or an immediate family member of the property owner, must live in either the primary residence or the ADU.
  • Number of occupants: Up to six unrelated persons may live in the primary residence and the ADU combined.
  • Subdivision: ADU units shall not be subdivided or otherwise segregated in ownership from the primary residence.
  • Size: The ADU unit shall be at least 200 square feet in size up to 800 square feet in size, provided that it does not exceed 40 percent of the total square footage of the primary residence. Garage areas are not counted in the calculations.
  • Scale: Detached accessory structures may be up to 25’ in height. The primary residence may be up to 35’ in height.
  • Location: The ADU unit may be created by 1) building an addition onto the primary residence 2) including it within the residence or 3) be a separate structure.
  • Front doors: The ADU shall have its own front door, however, only one front door may face the street. On corner lots, one door may face each street.
  • Architectural design: Any additions or new structures must meet current single family design standards and be consistent in style and appearance with the primary structure.
  • Parking: If legal on-street parking is not available immediately abutting the site, one additional parking space shall be required on site for the ADU.

Image courtesy

Most single family residential lots in the City are at least 7,200 SF which generally provides ample room to construct a detached ADU in the back yard. Depending on the size and material, a small ADU can easily be constructed for somewhere around $100,000. There are also companies in the area that provide prefabricated cottages (like Backyard Box, pictured above) for that same price range.

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